The present invention relates generally to data storage systems and, more particularly, to a system and method for estimating a frequency of slider airbearing resonance.
Within the data storage system manufacturing industry, much attention is presently being focused on reducing head-to-disk clearance as part of an effort to increase the storage capacity of data storage disks. It is generally desirable to reduce the head-to-disk clearance in order to increase the readback signal sensitivity of the transducer to typically weaker magnetic transitions associated with higher density disks. When decreasing the head-to-disk clearance, however, the probability of detrimental contact between the sensitive transducer and an obstruction on the disk surface significantly increases. As head-to-disk clearance continues to decrease, it becomes increasingly important to assess the general health of each read/write head, including flying characteristics, during the operating life of a data storage system.
A prevalent surface irregularity that afflicts an appreciable percentage of conventional data storage disks is generally referred to as an asperity. Asperities are isolated submicron-sized particles, typically comprising silicon carbide material, that are embedded in the disk substrate. No single mechanism has yet been identified as the source of such asperities, and it is believed that asperity defects arise from numerous sources. Such asperities are often large enough to interfere with the flight path of a typical slider/transducer assembly by physically impacting with the slider/transducer assembly at a very high velocity.
Further, asperities arising from the surface of a data storage disk are generally distributed in a highly random manner, and change in shape and size in response to changes in disk and ambient temperatures. A collision between a slider/transducer assembly and an asperity often renders the location of the asperity unusable for purposes of reading and writing information. Moreover, repeated contact between the slider/transducer assembly and asperity may cause damage of varying severity to the slider/transducer assembly.
Magneto-resistive (MR) transducers, for example, are particularly susceptible to interference from contact with asperities. It is well-known that MR transducers are very sensitive to variations in temperature, and are frequently used as temperature sensors in other applications. A collision between an MR transducer element and an asperity results in the production of heat, and a corresponding rise in transducer element temperature. Such transient temperature deviations are typically associated with an inability of the MR transducer element to read previously written data at the affected disk surface location, thereby rendering the stored information unrecoverable. An increase in the frequency of head-to-disk contact events may be indicative of a head that is flying lower than its intended average flyheight.
In the continuing effort to minimize head-to-disk clearance, manufacturers of disk drive systems recognize the importance of detecting changes in the flying characteristics of each individual read/write head during manufacturing and, importantly, during use of the disk drive system in the field. There exists a need in the data storage system manufacturing community for an apparatus and method for detecting changes in head flyheight. There exists yet a further need to provide such an apparatus and method which is suitable for incorporation into existing data storage systems, as well as into new system designs, and one that operates fully autonomously in-situ a data storage system. The present invention is directed to these and other needs.
The present invention is directed to a method and apparatus for estimating the value of a resonance frequency of an airbearing associated with a slider flying in proximity to a data storage medium. Estimating an airbearing resonance frequency according to the present invention involves obtaining a readback signal from a data storage medium over a plurality of complete airbearing periods and estimating the value of an airbearing resonance frequency using the readback signal information.
In accordance with one embodiment, a discrete signal segment comprising a plurality of frequency transform components is produced using the readback signal information, and the value of the airbearing resonance frequency is estimated using spectral leakage in the discrete signal segment. A number of different frequency transform techniques may be employed, including Discrete Fourier Transform (DFT), Fast Fourier Transform (FFT), and Short-Time DFT (STFT) techniques, for example. One of several frequency transform approaches may be implemented depending on whether the detected airbearing signal is stationary or non-stationary over time.
In accordance with another embodiment, a ratio of the magnitude of a first DFT component of the discrete signal segment to the magnitude of a second DFT component is computed at each of a plurality of sampling rates. Each of these sampling rates is defined by a number of samples per average airbearing cycle multiplied by a frequency falling within a range of expected airbearing frequencies associated with a given design or implementation. The second DFT component is related to the resonance frequency of the slider airbearing, and the first DFT component is a DFT component preferably adjacent to the second DFT component. The first DFT component may alternatively be a DFT component non-adjacent to the second DFT component. The resonance frequency value of the slider airbearing may be estimated using a minimum of the ratios. According to another embodiment, the minimum of a number of first and second DFT component power ratios may be used to estimate the value of the airbearing resonance frequency.
The discrete signal segment is preferably produced in response to detecting contact between the slider and a feature protruding from a surface of the data storage medium. The readback signal may comprise a magnetic signal component, a thermal signal component, or magnetic and thermal signal components. Other signal forms, such as optical signals, may also be processed by a method and apparatus according to the present invention. Goertzel""s algorithm may be employed to compute the magnitudes of the first and second DFT components of the discrete signal segment.
A method and apparatus for estimating a resonance frequency of an airbearing according to the principles of the present invention may be implemented in a data storage system and, preferably, implemented in-situ a data storage system without resort to circuitry external to the data storage system.
The above summary of the present invention is not intended to describe each embodiment or every implementation of the present invention. Advantages and attainments, together with a more complete understanding of the invention, will become apparent and appreciated by referring to the following detailed description and claims taken in conjunction with the accompanying drawings.